Transmission optical filters are used in spectroscopic applications to select a wavelength or a band of wavelengths of light emitted by a sample, and/or to select a wavelength or a band of wavelengths of light illuminating the sample. For example, in a fluorescence spectroscopic application, a beam of excitation light illuminates a sample, and light at a longer wavelength is detected to obtain its optical spectrum and/or to determine a total level of fluorescence emitted by the sample in response to excitation by the excitation light.
A single bandpass transmission optical filter can be used to measure the total level of fluorescence. The fluorescence levels measurement can be used to determine a concentration of fluorophore molecules, pH level, and the like. The fluorescence measurement can also be used to evaluate a concentration of non-fluorescent target molecules in a sample, by providing fluorophore molecules designed to change their fluorescence properties upon binding to the target molecules. The sample containing the fluorophore and target molecules is illuminated with the excitation light, and the optical power level of the fluorescent light is measured.
A typical spectrofluorometer suitable for the above purpose is shown in FIG. 1. The spectrofluorometer 10 includes a light source 11, collimating/focusing lenses 12A, 12B, and 12C, an excitation filter 13, a fluorescence filter 14, a beamsplitter 15, and a photodetector 16. In operation, the light source 11 emits excitation light 17 shown with solid lines. The excitation light 17 is collimated by the leftmost lens 12A, filtered by the excitation filter 13, transmitted through the beamsplitter 15, and is focused by a rightmost lens 12B onto a sample 18. The illuminated sample 18 emits fluorescent light 19 shown with dashed lines. The fluorescent light 19 is collimated by the rightmost lens 12B, reflects from the beamsplitter 15, and is focused by the bottom lens 12C onto the photodetector 16. The excitation and fluorescence filters 13 and 14 are multilayer dielectric filters, which are preferred over other types of filters for their good wavelength selectivity and a comparatively low optical insertion loss.
The spectrofluorometer 10, although widely used, has a drawback of a relatively large size. Nowadays, miniature fluorometers can be used in implantable glucose measurement probes deployed subcutaneously, that is, under skin of human patients. For example, Senseonics Inc. of Germantown, Calif., USA, developed a continuous blood glucose monitoring system intended for diabetes patients. The monitoring system includes a subcutaneous probe having a miniature fluorometer as a blood glucose sensor.
Due to subcutaneous placement of the spectrofluorometer, the latter needs to be made as small as possible. It is a goal of the invention to provide filters which enable a compact spectrometer assembly suitable for under-skin placement. Other numerous applications of miniature spectrometer assemblies using these filters are of course possible.